diff --git a/.readthedocs.yml b/.readthedocs.yml
index 27a6134..e1f183f 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -1,6 +1,7 @@
 version: 2
 
 build:
+  os: ubuntu-20.04
   tools:
     python: "3.9"
 sphinx: